While this information is somewhat buried in the depths of the FAQ, I thought I'd highlight a point from How to Answer:
Answer the question
Read the question carefully. What, specifically, is the question asking for? Make sure your answer provides that – or a viable alternative. The answer can be “don’t do that”, but it should also include “try this instead”. Any answer that gets the asker going in the right direction is helpful, but do try to mention any limitations, assumptions or simplifications in your answer. Brevity is acceptable, but fuller explanations are better.
The way I interpret this is that if someone asks how to do X in situation Y, it's perfectly acceptable to explain that it's not a good idea and why, and then explain an alternative and explain why the alternative is better.
This happens all the time on Stack Overflow. For instance, if someone asks a question about how to properly format chat messages received in a Web based chat, and I see they're using polling, I'll let them know that polling just isn't really the way to go. As an aside, I'll provide an answer to their questions as it applies to polling, assuming there is an answer, but I'm always sure to explain why it's just not a good idea.
I see nothing wrong with this approach.
However, if someone didn't address the full question, then we may want to suggest they add more information. I do this by asking follow up questions to the answerers, but the follow ups are taken from the question itself. This is one way we can get people to add more to their answers without putting them on the defensive.
Also, in some other instances, there's an implied answer. For example, one answerer explained that vacation time is an entitlement and part of your pay, but he didn't explicitly say, "So take your darn vacation already :)". So, I just added that part in as an edit so it was clearer to others that he was saying the asker should take his well-deserved vacation.
Lastly, I feel it's difficult to discuss this without specific examples. We discuss specific questions here all the time, so feel free to bring in examples of answers as well. As long as we do it in a respectful manner, we should be able to have an effective discussion about what we can do better. Hope this helps! :)